1. Field of the Invention
This invention pertains generally to methods for fabricating three dimensional objects, and more particularly to methods for generating such objects using stereolithographic techniques.
2. Description of the Background Art
Stereolithography is a technique that was originally developed for rapidly producing prototypes of three-dimensional physical parts that are designed using computer-aided design (CAD) methods. This technique is basically a "three-dimensional printing process" that produces a solid plastic model of the CAD part by using a laser beam to draw cross sections of the part on the surface of a photo-curable liquid plastic or resin. The apparatus for carrying out this method, termed Stereolithography Apparatus (SLA), includes a computer program for slicing the CAD model into thin cross sections. One embodiment of this method includes focusing a laser beam to a small intense spot on the surface of the liquid resin by means of a computer-controlled optical scanning system. The beam cures liquid resin to a solid at the locations on the surface corresponding to the cross section of a given layer in the CAD model. A computer-controlled elevator system then lowers the newly-formed layer into the liquid for a distance comprising the layer thickness, and another layer is formed on top in a similar manner, such that each layer adheres to the layer below it. The solid model is thus built up, layer by layer, from successive cross sections of the CAD model, starting from the bottom up to the top of the model. The entire process is controlled by a computer, which effectively converts the CAD data for a physical three-dimensional object into a solid model realization.
This technique has been described in detail in the technical literature, including a series of patents that disclose this technique and associated SLA devices. Reference is made to U.S. Pat. No. 4,575,330 (Hull), issued Mar. 11, 1986, which is one of the earlier patents describing this technology. This technique has been further discussed extensively in the text entitled Rapid Prototyping and Manufacturing; Fundamentals of Stereolithography by Paul F. Jacobs, published in 1992 by the Society of Manufacturing Engineers. The foregoing references are incorporated herein by reference. Apparatus for carrying out this technique is currently manufactured and sold by 3D Systems, Inc. of Valencia, Calif.
Previously, the known stereolithographic systems have been used to fabricate relatively large prototype parts of limited precision. The above-identified reference, "Rapid Prototyping and Manufacturing; Fundamentals of Stereolithography", p. 314, reports that the overall dimensional accuracy of prototype parts fabricated by SLA approaches 5 mils (0.005 inches), compared with the CAD model values. This 5-mil resolution value is the extreme limit, and SLA-fabricated parts more typically have a resolution in the 10 mil region. The SLA system manufactured by 3D Systems, Inc. produces parts in which the minimum horizontal feature size is limited to about 10 mils, and the laser beam positioning accuracy is limited to 5 mils (SLA-250 User's Manual, Page B-1). Therefore the stereolithography technique that is known to persons of ordinary skill in the art is unsatisfactory for production of very small parts with high precision, where the above resolution limits exceed the acceptable tolerances for such high precision parts. It is desirable to adapt and modify the known stereolithography techniques for fabricating small parts with accuracies and tolerances at least 10 times smaller than the above limits (i.e. 10 times higher precision).